Although it's very hard to calculate the effective efficiency of my LF antenna, I've tried an indirect
evaluation, with the aid of the german station DCF39, transmitting on 139 Kc, and starting with a
careful calculation of my single-turn receiving loop.(see pictures)

The first step has been the definition of the parameters of the single-turn alone, supposed
in the free space, subsequently, using the Mininec method, it was modeled in its operative
position accounting for the presence of the building and for the surrounding heavy urbanized terrain.
Remember that in the layout at right, T1 is the toroidal transformer, 2 / 18 t., located at the
base of the loop, L is the coaxial line, C the tuning condenser, A the safety relais, T2 the output
transformer and O the output connector.

With the Mininec I've found about the same efficiency in free space and, over ground, values
ranging from - 61 to - 59.5 dBi at some degrees of elevation with a strange peak of - 45.5 dBi at
42 degrees.These values are obviously approximate, being very difficult to reproduce exactly the
real conditions of operation, particularly the strange peak at 42 degrees appears suspicious.

The loaded Q measured at points 1, 2 (see layout), results = 76, so the whole loop, considering
the turn-ratio of the toroidal transformer and the gain of the amplifier, shows a total
efficiency of - 38.13 dBi in free space and, following the results of Mininec calculation,
(excluding the strange peak at 42 degrees), from - 33.7 to - 33.2 dBi in its operative position.

These values seem to be roughly confirmed by the signal level of the german
station DCF39 transmitting from Burg, about 950 km away, with an ERP of 40 Kw = 76 dBm.
This level varies daily between - 70 and - 40 dBm, reaching frequently - 35 dBm.
A careful calculation of the ground wave gives a field strength around 100 uV/mt, an estimation of
space wave gives around 180 uV/mt, supposing zero losses in the ionosphere, the space wave could
reach 360 uV/mt. Adding this last value to the 100 uV/mt of the ground wave (supposing both waves
arriving in phase), and relating the resulting field strength with the level of - 35 dBm, read on my
receiver, the efficiency of the loop would be around - 28 dBi, probably a too high value, so seems
to be more realistic suppose a further increase of the space wave due to a concentration in the
reflection of the ionosphere, and confirm the efficiency of the loop around - 33 - 34 dBi.

At this point, comparing the signal received by the loop with the signal received by the antenna,
this last appears in substantial parity with dry weather and about 2 dB lower with wet weather.
So, averaging, the efficiency of the antenna can be estimated around - 34 - 35 dBi, meaning an
EIRP of roughly 80 - 60 mW with 200 W applied power.

This value appears well higher in respect to the 20 - 30 mW estimated before, but really there has
been a noticeable improvement, after the last modifications made in the antenna circuitry, as
confirm the very good reports received from DX stations.